Method and apparatus for burning liquid fuels



4 Sheets-Sheet l ALBERT E. WELLER,JR. GALE R. WHITACRE IN VEN TORS June 20, 1967 Filed June 17. 1964 June 20, 1967 A. E. WELLER, JR., ETAL 3,326,262 METHOD AND APPARATUS FOR BURNING LIQUID FUELS Filed June 17, 1964 l 4 sheets-sheet a ELECTRIC 45 HEATER ALBERT E. WELLER, JR.

GALE R. WHITACRE INVENTORS June 20, 1967 A. E. WELLER, JR., ETAL 3,326,262

METHOD AND APPARATUS FOR BURNING LIQUID FUELS 4 Sheets-Sheet 5 Filed June 1'?, 1964 BURNER B BALANCE POINT BURNER A BALANCE POINT 5 BURNER-ON TIME MINUTES Fig. 6

BURNER u ON ALBERT E. WELLER, JR.

\ sTANoBY BURNER OFF COOLING TIME Fig. 3

VII

GALE R. WHITACRE INVENTORS sgi/7, wm, @ai

June 20, 1967 A. E. wELLER, JR.. ETAL 3,326,262

METHOD AND APPARATUS FOR BURNINQ LIQUID FUELS 4 Sheets-Sheet 4 Filed June 17, 1964 3.o IN.

METAL HEAT-STORAGE VOLUMEI IN.3

Fig. 5

R. nu R E L W E T R E B L A 0.v 0 o... IO m O, mm 4 m ml A/ .Y H 0 F IIOI m m. w a

GALE R. WHITACRE INVENTORS 74%MQ Ma ,Oa/wom l I 4D IO VAPORIZATION RATE, GIIL./I'IR.FT.2

HEAT FLux, BTU/infn2 l* I I 0.04 o.| 0.4

Fig. 4

United States Patent O 3,326,262 METHOD AND APPARATUS FOR BURNING LIQUID FUELS Albert E. Weller, Jr., and Gale R. Whitacre, Columbus,

Ohio, assignors, by mesne assignments, t American Petroleum Institute, New York, N.Y., a corporation of the District of Columbia Filed June 17, 1964, Ser. No. 375,753 The entire term of the patent has been dedicated to the Public 12 Claims. (Cl. 158-5) This invention relates to a method and apparatus for vaporizing and burning liquid fuels. The invention is more particularly concerned with method and apparatus for maintaining vaporizing conditions for fuel while minimizing the formation of carbon from the fuel vaporization. The invention is especially suitable for use in the vaporization and burning of 4the distillate fuel oils that customarily form excess carbon deposits when conventional vaporizing and burning processes are used.

There are many examples of vaporizing burners in the art. The most frequently used type of vaporizing apparatus is an open pan positioned in the combustion chamber that receives the fuel for vaporization. After the burner is once started, the heat from combustion vaporizes the fuel for continued combustion. However, when the burner is rst started-up the vaporizing surface is cold and the vaporizing surface also cools during the periods when the burner is off. Vaporization of only the l-ow boiling fraction occurs until the burner heats the vaporizing surface .to the proper temperature. The problem of start-up vaporization is overcome in some instances by providing heating means, such as a primer llame or an electric coil, -to heat the vaporizing surface until such time as the surface may be heated by the combustion of the burner. If the liquid fuel is of a type that tends to produce carbon when burned in the liquid form, the disadvantages of a primer llame are obvious. A heating coil either operates on a stand-by basis during burner-olf periods or starts heating the vaporizer just before start-up, in which case there is a time delay between each cycle before the burner is turned on. Since most burners operate with an on-period, off-period cycle, the stand-by temperature of the vaporizing surface becomes a problem.

A primary object of this invention is to provide a vaporizing surface that receives heat from the combustion of the burner, utilizes a portion of the heat to vapo-rize the fuel, and stores a portion of the heat to vaporize fuel for the next burner start-up.

Another object of this invention is to provide a partial one-way heat transfer from the combustion chamber of the burner to the vaporizing surface, so that the vaporizing surface receives large amounts of heat during combustion but allows very little heat to be returned to the combustion chamber when the burner is not operating.

This invention is especially adapted for burning the distillate fuels that usually form large amounts of carbon deposits when the more conventional types of vaporization processes are used. Recent advances in the combustion of distillate fuels indicate that such fuels may be vaporized with less carbon deposits if the temperature of the vaporizing surface is relatively high. Another important factor is the heat flux supplied by the vaporizing surface to the fuel. The actual temperature of the oils during vaporization may remain at about 520 F., even though the B.t.u. per hour supplied across a given amount of vaporizing surface may be increased considerably, The formation of carbon deposits depends on the residence time of the oil, i.e., the amount of time elapsed between the introduction of liquid fuel onto the vaporizing surface and its transformation into the vapor phase.

Accordingly, another object of this invention is to provide and maintain a heat flux to the vaporizing surface and consequently to the fuel oil in amounts suflicient to reduce residence time and thereby minimize deposits of carbon. Fuel rates, heat transfer rates, and carbon deposit rates are interdependent and the selection of various parameters for the preferred results and conditions are discussed subsequently herein.

Still another object of this invention is the continual removal of traces of carbon deposits preferably by introducing oxygen-containing gas such as air onto the hot vaporizing surface thereby removing carbon deposits by oxidation.

Briefly, this invention includes a method for burning liquid fuel. The liquid fuel is introduced onto a heated surface with the heated surface being at a first temperature that is sufficient to vaporize the fuel. The vaporized fuel is then transferred to -a combustion area, mixed with air, and then burned. A portion of the combustion heat is returned or transferred to the vaporizing surface and the temperature of the vaporizing surface is elevated to a second temperature to store a portion of the transferred heat. When the introduction of liquid fuel is interrupted and combustion is suspended, the heat loss rate from the vaporizing surface is controlled so that the time required for losing the amount of heat gained is greater than the time required to gain the heat, thereby providing a standby period wherein the vaporizing surface is maintained above the rst temperature.

One advantage of this invention is that a method and apparatus are provided for burning liquid fuel wherein a vaporizing surface is maintained at a vaporizing temperature such that the carbon-forming tendency of vaporizlng liquid fuel is minimized.

Another advantage of this invention is that the mamtenance of the vaporizing surface at the vaporizing temperature is economical due to storage of regenerated heat and the control over heat loss rates thus obtaining economical use of power in the `form of heat.

Still another advantage of this invention is that the distillate fuel oils, previously prohibitive for use in vaporizing burners `due to carbon formation, which require frequent cleaning, are useable.

Still other objects and advantages of this invention will be `apparent from the description that follows, the drawings, and the appended claims.

In the drawings:

FIG. 1 is a sectional elevational view of a vaporizing burner constructed according to the invention;

FIG. 2 is a sectional elevational view of a second Vaporizing burner constructed according to the invention;

FIG. 3 is a graph of the cyclic operation of a burner showing the relationship of time versus vaporization temperature;

FIG. 4 is a graph showing the relationship of heat ilux versus carbon deposits;

FIG. 5 is a graph of vaporizer metal heat-storage volurne versus standby heat loss and the effect of insulation and purge .air to the vaporizer; and

FIG. 6 is a graph showing the critical load factor curve for two-example burner constructions.

lFIG. 1 shows a sectional view of an `oi-l burner 21 for the thermal rvaporization of liquid fuel and is especially adapted for use lwith the distillate liquid fuels, for example, No. 2 heating oil. The apparatus is constructed to deliver heat to a vaporizer 23 at higher heat flux during burner-on periods than could be obtained directly with flame. The vaporizer 23 is preferably a casting with the bottom thicker than the walls.

The required heat flux is obtained by providing a combustion chamber 25 that is lined with a refractory 27 that becomes incandescent when heated. A heat shield 29 is preferably placed between the refractory 27 and a housing 31. During combustion, when the refractory 27 becomes incandescent, heat is radiated to the exposed surface 33 of the vaporzer 23 as indicated by the wavey arrows 35-35. As will be seen in FIG. 1, the exposed surf-ace 33 is shielded in the ceiling of the combustion chamber from convection currents during vburner-off periods by being in a recess 37 in the insulation 39 surrounding the vaporzer 23.

The vaporzer 23 is provided with a temperature sensing element 41 which is connected to a thermoswitch 43. If the temperature of the vaporzer 23 cools to a selected minimum value, the thermoswitch 43 activates an electric heater 45 which maintains the vaporzer temperature Aat or above the selected minimum temperature. In the case of distillate liquid fuels the preferred vaporizer minimum temperature is about 750 F. The electric heater 45 is also activated on initial start-up (after the burner has been shut down completely fora period of time) to bring the vaporzer 23 up to the vaporizing temperature.

Liquid fuel 46 enters the vaporzer 23 by means of a fuel supply duct 47 and the fuel How rate is controlled by a fuel metering device 49 which in turn receives fuel from an oil supply (not shown) through an oil inlet 51. The fuel supply duct 47 is surrounded by an air tube 53 that communicates between a blower 55 and the vaporizer 23. Air passes from the blower 55 thro-ugh the air tube 53 (as .shown yby the arrow 57) during blower operation. The air keeps the fuel duct 47 cool and oxides away the trace carbon deposit in the vaporzer 23 when fuel ow is interrupted. The fuel duct 47 is kept cool in order to insure that carbon deposits do not form and block the fuel flow. The vaporized fuel is transferred from the interior 59 of the vaporizer 23 through a vapor-exit tube 61 (in a direction shown by the arrow 63) into the combustion chamber 25.

Combustion air is supplied from the blower 55 to a port 65 in the combustion chamber 25. The combustion air port 65 is adjacent to an opening 67 of the vapor-exit tube 61. An igniter 69, preferably a spark type ignition, is provided at the ports 65 and 67 to ignite the vaporized fuel and air mixture. The combustion gases are released through a ue 71 to a heat receiver or heat exchanger (not shown). When the fuel supply to the vaporizer 23 is shut off, a damper 73 is closed shutting off the combustion air supply to the combustion chamber 25 but still providing a Asmall flow of air through the air tube 53. The damper 73y is an additional provision to ensure slow cooling of the vaporzer by eliminating unnecessary air circulating through the combustion chamber 25 and possibly contacting the exposed vaporizer surface 33.

FIG. 2 shows a sectional view of a second embodiment of an oil burner 21 for the thermal vaporization of liquid fuel in accordance with the method herein described. The

,high heat gain versus low heat loss to the vaporzer 23 is accomplished by a thermosiphon principle.

The combustion gases in combustion chamber 25 supply heat to a metallic tube 81 which is shown in FIG. 2 as U-shaped. Other shapes, for example a coil, are also possible. The metallic tube `31 extends through the refractory lining 27 and communicates with a container 83. The container 83 encloses the vaporzer 23. Both container 83 and tube 81 are filled with a heat transfer material 85 such as lead. The vaporzer 23 is completely surrounded bythe material v85. The preferred material is lead `containing an oxygen gettering agent, such as magnesium, and corrosion inhibiting agents such as titanium and zirconium. Other possible materials include leadfbismuth alloys, bismuth, or sodium with suitable gettering and inhibiting agents. The chamber 83 and tube 81 are charged with liquid lead and the chamber 83 purged with argon. The 'additives to the liquid lead and the argon purge aid in preventing oxidation and corrosion at the elevated temperatures of vaporzer operation.

At initial start-up, the electric heater 45 (having coils 87 surrounded by the material 85) heats the material 85 (and, in the case of lead changes it to a liquid). The material `85, in turn, heats the vaporzer 23. When the vaporizing temperature of the liquid fuel is attained, liquid fuel 46 is supplied through the fuel supply duct 47 and is vaporized. The vaporized fuel is transferred through a coiled vapor superheater tube 89, immersed in the material 85, through a vapor-exit tube 61, to a port 67 and a burner 91.

Combustion air is supplied tangentially to the burner 91 from an air supply source (not shown) through an air duct 93 and enters a port `65 near the vapor-exit tube port 67. The vaporized fuel and air mixture is ignited by an ignitor (not shown) and the combustion products supply heat through radiation and convection to the tube 81. The material thermosiphons and carries heat to the vaporizer 23 in the container 83. When combustion is suspended the thermosiphon action stops and, in effect, insulates the vaporzer 23.

The air supply (not shown) is also connected to the air tube 53 thereby providing air for cooling the liquid fuel 46 during fuel flow, and additionally, for oxidizing trace carbon deposits in the vaporzer 23 during burner-off periods.

The vaporizer 23 shown and the associated structure (thermosiphon and exposed surface) are called a heat flux transformer. The term is very appropriate since the combustion heat is gathered from a large surface and applied to a smaller surface (surface 25 to 33 in FIG. 1 and the material 85 to the vaporzer surface in FIG. 2).

FIG. 3 is a graph showing a typical operating cycle for the lburners 21 and 21' of FIGS. l and 2 using No. 2 heating oil. The temperature of the vaporzer 23 is plotted against time. The start-up phase is not shown on the graph of FIG. 3; a portion of a number of successive cycles is shown by the curve. The vaporizer 23 is already heated to about 750 F. The burner (21, 21') comes on and heats.

the vaporzer 23 to a temperature in excess of 750 F. When the heating conditions of the particular application for the burner (21, 21') are fulfilled, the fuel ow is interrupted and the vaporzer coasts7 or enters a cooling period. If the vaporzer temperature falls to about 750 F., the electric heater 45 is activated and the stand-by phase of the cycle is operative. Under the most common heating conditions, however, the particular application for the burner (21, 21') will require a burner-on period before the vaporizer 23 has cooled to about 750 F. For other liquid fuels the vaporzer standby temperature may be different than 750 F.

FIGS. 4 and 5 are intended to show various relationships between some of the elements and parameters that are considered in the method and apparatus of this invention. FIG. 4 is a graph showing the carbon deposit rate 1n `grams per thousand gallons of No. 2 heating oil plotted against heat flux in Btu. per hour for a square foot of transfer surface. The rates are compared for three different film thicknesses of oil and No. 2 heating oil is selected for examination since it forms carbon deposits easily. The heat flux 1s also proportional to the vaporization rate shown in gallons per hour for each square foot of vaporizing surface. It should be noted that the relationship between heat flux and carbon formation is a hyperbolic relationship and when plotted on a log-log scale the carbon deposits and heat ilux form a straight line relationship. Also if the lm is kept rather thin, the carbon deposits are reduced. If a temperature of about 750 F. is selected for the vaporzer surface, the area of the vaporzer surface is adequate, and fuel supply rate is balanced with respect to the surface area to keep the `'llm on the vaporzer thin, thus, if the temperature is too far below 750 F. the area for the vaporizing surface becomes impractically large. We have operated specific examples of burners shown in FIGS. 1 and 2 at fuel rates of 0.25 to 0.5 gallon of fuel per hour and carbon deposits have been less than 10 grams per 1000 gallons of fuel. The addition of purge air into the vaporizer 23 aids in keeping the carbon deposits even lower; an equilibrium state is reached when carbon deposits are continually oxidized away almost as fast as they appear leaving only about l gram of deposit per 1000 Igallons of fuel vaporized.

The relationships shown in the graph of FIG. 4 indicate that there is a direct relationship between heat flux, film thickness and carbon deposits. Actually, there are a few subtilties not made obvious `by the curve. It is possible to vaporize No. 2 heating oil at a temperature lower than 750 F. by simply Imaking the oil lm very thin and providing a faster rate of heat transfer. However, oxidation of the trace deposits of carbon requires a temperature of about 750 F. so that carbon burn-off is the real limitation on temperature. Also, the lm cannot become too thin. Mixing new fuel with the vaporizing fuel prevents fractionation of the fuel. Thus the steady addition of fresh fuel to the vaporizing fuel provides a well mixed fuel keeping the heavier portions of the fuel from depositing as a residue. The fuel oil in the vaporizer actually sputters and spins due to the temperature of the vaporizer surface and thus the incoming fuel is well mixed. The true measure of carbon formation is the residence time of the oil. The following table is an example of some interrelations from FIG. 4 for an oil depth of 0.1 inch:

Oil Temp., Vaporization Heat Flux, Deposit Residence F. R e, B.t.u./hr.-ft.2 Formation, Time, Sec.

gal./hr.ft.2 g./1,000 gal.

It should `be noted that residence time and carbon deposit decrease as heat flux is increased. The film thickness divided bythe heat flux gives a value wherein equal quotients of film thickness and heat flux correspond to equal residence times. Residence time of the fuel in burners 21 and 21 is between about 5 to 10 seconds and customarily it is near five seconds. The quotient of film thickness and heat liux that results in proper residence time is about 2 l0"6 or less.

Some heat, during burner-off periods, will be lost from the vaporizer 23. Some will be lost through the entrance and exit tubes, some will be lost back into the combustion chamber 25. Heat loss also depends on such factors as the storage volume of metal in the vaporizer 23, the thickness of the insulation 39 that surrounds the vaporizer 23 and the flow rate of the purge air supplied through the air tubes 53. FIG. 5 is a graph of metal heat-storage volume in cubic inches versus the standby heat loss in watts when the vaporizer is at 750 F. The losses for the all-steel vaporizer 23 in burner 21 of FIG. l and the thermosiphon unit in burner 21 of FIG. 2 are both shown. Each curve has a designated insulation thickness in inches with the solid lines representing purge air at 0.06 cubic feet per minute and the dotted lines representing purge air at 0.12 cubic feet per minute. The noticeable features of the curve are that an increase in insulation thickness and decrease in purge air reduce heat losses from the vaporizer 23. The volume of metal selected for the vaporizer of a particular burner will depend on the amount of fuel to be vaporized and the heat flux desired. The fuel rate will, of course, depend on the desired capacity for the heating application. It should be noted from the foregoing, then, that the selection of various parameters will result in a burner having characteristics dependent on such selection. For example, the size or watt output of the electric heater 45 for initial start-up and stand-by heat will be largely dependent on the graph shown in FIG. 5. The electric heater size will also depend on the rate desired for bringing the cold vaporizer up to starting temperature. If the watt output of the electric heater 4S is only a little greater than heat loss rate of the vaporizer the warm up time of the vaporizer 23 will be impractically long; however, on stand-by operation the output need only `be slightly greater or equal to heat loss from the vaporizer 23.

The following table gives data for two burners, one constructed in accordance with FIG. l and the other in accordance with FIG. 2.

In the selection of the a-bove parameters, the primary factor to consider is ensuring that the heat flux is sufficient to vaporize the fuel with minimal carbon deposition, preferably below 5.0 grams per 1000 gallons of fuel. Other factors are selected according to service requirements and performance desired.

One performance characteristic involves critical load factor (hereinafter designated CLF for brevity). As previously mentioned, most service requirements require a cyclic operation of burner-on periods and burner-off periods. If the burner is to operate economically, the burner should be self-sustaining over the major range of heating loads, i.e., the vaporizer temperature should stay above the selected vaporizing temperature without activation of the electric heater. In space heating for example, burner-off periods will be quite long in mild weather and decrease as the weather becomes colder.

The CLF is defined as the minimum percentage of burner-on time which just sutlices to provide for selfheating of the vaporizer 23. The CLF is the percentage of 'burner-on time in terms of total cycle time (burneron time plus burner-off time) for return to the selected temperature for the vaporizing surface (with No. 2 heating oil, about 750 F.). To obtain the most favorable or lowest CLF, the initial rate of temperature rise should be made as high as possible. This requires the largest regeneration rate possible within the limits imposed by permissible metal temperature. The amount of heatstorage metal used in a particular construction also affects the CLF.

In home heating applications, the percentage of burneron time is determined by the heat load, but the duration of each firing cycle is determined almost entirely by the characteristics of modern room thermostats employing heat anticipation. FIG. 6 shows a characteristic curve for a typical room thermostat. The thermostat characteristic results in a minimum burner-on time of 3 minutes, and equal on and off times of 5 minutes at a 50 percent load factor.

For a particular burner, the CLF for self-heating appears as a curve. The CLF curves for the specific burners A and B given in the table above are shown on the graph of FIG. 6. The slight lag in regeneration rate of lburner A is due to the lag in temperature rise of the incandescent refractory lining the walls of burner A. Operation to the left of a CLF curve requires supplemental heat (from the electric heater). For operation to the right of a CLF curve, the vaporizer temperature never falls below the selected temperature of 750 F. and supplementary heat is not needed.

As the weather alters the home-heating load, a burner operates with on and off periods according to the thermostat characteristic curve. The critical points for self heating are located on FIG. 6 where the thermostat characteristic curve intersects the CLF curve for a specific -burnenThe various values for load factors are shown by the dotted lines. For burner A the critical point is about '16 percent as read from the fan-shaped family of `dotted lines that indicate the actual load factors (burner `of time percentages) to supply a given load. Burner B has a critical point of approximately percent. Thus, vat loads above these critical points (toward the right in FIG. 6) no supplementary heat is required. For example, if burner A is fired for 3 minutes (the minimum) it will be 151/2 minutes before the vaporizer will cool to 750 F. and the auxiliary heater 45 will require activation.

The method of the invention, in accordance with the foregoing description, involves introducing a liquid fuel onto a vaporizing surface elevated to a temperature above the vaporization point of the fuel (about 750 F. in the case of No. 2 heating oil), regeneratively heating the vaporizing surface by combustion of the vaporized fuel (providing a fuel film thickness in inches which on being (divided by the heat flux in B.t.u./hr.-ft.2 gives a quotient of less than about 2 10-6 in the case of No. 2 heating foil, and insulating the vaporizing surface during noncombustion periods, thereby providing a CLF of about 15 percent) to maintain the vaporizing surface above the fuel vaporization temperature.

It will be understood of course that, while the forms of the invention herein shown and described constitute preferred embodiments -of the invention, it is not intended to illustrate all possible forms of the invention. It will also be understood that the words used are words of description rather than of limitation and that various changes may be made without departing from the spirit and scope of the invention herein disclosed.

What is claimed is:

1. Apparatus for burning liquid fuel, comprising:

(a) a combustion chamber;

(b) an insulating material adjacent to said combustion chamber;

(c) a refractory material lining said combustion chamber, said refractory material becoming incandescent during combustion;

(d) a vaporizer chamber having an exterior surface exposed in the ceiling `of said combustion chamber to radiations from said refractory material during combustion and isolated from convection currents in said combustion chamber by being recessed into said insulating material and with the remaining exterior surfaces of said vaporizer chamber surrounded by said insulating material;

(e) a communicaiton passage from said vaporizer chamber to said combustion chamber;

(f) a fuel inlet from a fuel supply source communicating with said vaporizer chamber;

(g) Ia soruce of air communicating with said combustion chamber for supplying combustion air to mix with vaporized fuel from said vaporizer chamber; and

(h) ignition means positioned to ignite the mixture of vaporized fuel and air.

2. Apparatus for burning liquid fuel according to claim 1 wherein an electric heater is positioned to apply auxiliary heat to said vaporizer chamber.

3. Apparatus for burning liquid fuel, comprising:

(a) a combustion chamber;

(b) an insulating material adjacent to said combustion chamber;

(c) a first chamber surrounded by said insulating material, said chamber filled with a material having a liquid state at fuel vaporizing temperature, and said chamber having a loop passage, said loop extending into said combustion chamber whereby heat from said combustion chamber causes said material in liquid state to thermosiphon;

(d) a second chamber positioned in said first chamber for vaporizing fuel, said second chamber surrounded by said material having a liquid state at fuel vaporizing temperature;

(e) a communication passage from said second chamber to said combustion chamber;

(f) a fuel inlet from a fuel supply source communicating with said second chamber;

(g) a source of air communicating with said combustion chamber for supplying combustion air to mix with vaporized fuel from said second chamber; and

(h) ignition means positioned to ignite the mixture of vaporized fuel and air.

4. Apparatus for burning liquid fuel according to claim 3 wherein an electric heater is positioned to apply auxiliary heat to said material having a liquid state at vaporizing temperature.

5. Apparatus for burning liquid fuel according to claim 3 wherein said material having a liquid state at vaporizing temperature is lead.

6. Apparatus for burning liquid fuel according to claim 3 wherein said material having a liquid state at vaporizing temperature is sodium.

7. Apparatus for burning liquid fuel according to claim 3 wherein said material having a liquid state at vaporizing temperature is bismuth.

8. A method of burning fuel over a period of spaced intervals of time comprising the steps of:

(a) introducing liquid fuel onto a heated surface, said heated surface being at least at a first temperature sufficient to support oxidation of carbon and vaporization of liquid fuel;

(b) transferring the vaporized fuel to a combustion area;

(c) mixing the vaporized fuel with air in said combustion area;

(d) burning the vaporized `fuel and air mixture;

(e) transferring heat from the combustion area to said heated surface in quantities sufficient to elevate the temperature of said heated surface to a second temperature and store a portion of the transferred heat;

(f) interrupting the introduction of liquid fuel onto said heated surface, thereby interrupting the burning of the vaporized fuel and transfer of heat to said heated surface;

(g) retarding the loss of heat from said heated surface to the extent that the heat transfer rate from said heated surface during the interruption of liquid fuel introduction is substantially less than the heat transfer rate to said heated surface during the burning of the vaporized fuel and air mixture thereby providing a stand-by period wherein the heated surface has a temperature above said first temperature; and

(h) delivering air to said heated surface during said stand-by period to oxidize carbon and applying auxiliary heat to said heated surface when the temperature of said heated surface diminishes into coincidence with said first temperature during said stand-by period.

9. A method of burning fuel oil over a period of spaced intervals of time, lcomprising the steps of 2 (a) heating a surf-ace to a temperature of at least about 750 F. using a first heat source;

(b) introducing fuel oil onto said surface and allowing said fuel oil t-o vaporize;

(c) transferring the vaporized fuel oil to a combusvtion area;

(d) mixing the vaporized fuel with air in said combustion area;

(e) burning the vaporized fuel and air mixture to provide a second heat source;

(f) transferring heat from said second heat source to said surface in quantities sufficient to elevate the temperature of said surface above approximately 750 F. and to store a portion of the transferred heat;

(g) interrupting the introduction of fuel oil onto said surface thereby temporarily eliminating said second beat source;

(l1) retarding the loss of heat from said surface to the extent that the heat transfer rate from said surface during the interruption of lfuel oil introduction is substantially less than the heat transfer rate from said second heat source to said surface thereby prolonging the period of time during which said surface remains above approximately 750 F. after temporary elimination of said second heat source; and

(i) delivering air to said heated surface While the introduction of fuel oil onto said surface is interrupted and activating said rst heat source when the temperature of said surface falls to approximately 750 F.

10. In a method for separately Vaporizing and burning liquid fuel wherein the combustion -process supplies heat to a vaporization surface in the vaporizing process, the improvement of initially heating the vaporization surface up to within a liquid fuel vaporization temperature range, introducing liquid fuel to the heated vaporization surface and steadily adding and mixing fresh liquid fuel to the Vaporizing fuel and feeding off vaporized fuel from the vaporization surface to the combustion process While controlling the fuel lm thickness and heat iiux quotient at the vaporization surface to be the equivalent of a value of less than about 2 106 expressed for thickness of the film being in inches and the heat flux being in B.t.u.s per hour per square foot, interrupting the vaporization of liquid fuel, controlling the loss of heat from said heated vaporization surface to the extent that the rate of heat loss from said surface during the interruption of vaporization of liquid fuel is less than the rate of heat transfer to said surface during the combustion process, and introducing auxiliary heat to the heated vaporization surface when the temperature of said heated surface coincides with a minimum temperature in said vaporization temperature range during said period of noncombustion.

11. In a method for separately vaporizing and burning liquid fuel wherein the combustion process supplies heat to a vaporization surface in the vaporizing process, the improvement of initially heating the vaporization surface up to within a fuel vaporization carbon oxidation temperature range, introducing liquid fuel to the heated vaporization surface and steadily adding and mixing fresh liquid fuel to the vaporizing fuel and feeding off vaporized fuel from the vaporization surface to the combustion process while controlling the fuel hn thickness and heat flux quotient at the vaporization surface to be the equivalent of a value of less than about 2 l0F6 expres-sed for thickness of the film being in inches and the heat iiux being in B.t.u.s per hour per `square foot, in terrupting the vaporization of fuel for a period of noncornbustion, introducing purge air to the heated vaporization surface during the interruption of vaporization of liquid fuel and controlling the loss of heat from said heated vaporization surface to the extent that the rate of heat loss from said surface during the interruption of vaporization of liquid fuel is less than the rate of heat transfer to said surface during the combustion process,

and introducing auxiliary heat to the heated vaporization surface when the temperature of said heated surface 5 coincides With a minimum temperature in said fuel vaporization carbon oxidation temperature range during said period of noncombustion.

l2. Apparatus for burning liquid fuel comprising:

(a) a combustion chamber;

(b) a vaporization chamber in heat exchange relation to said combustion chamber and having an outlet in communication with said combustion chamber;

(c) a fuel inlet communicating with said vaporization chamber and with a source of liquid fuel supply and having a fuel control to open and close said fuel inlet, for said vaporization chamber heated to within a fuel vaporization and carbon -oxidation temperature range to receive liquid fuel from said source of supply and give vaporous fuel when said fuel inlet is open and for liquid fuel from said source of supply to be interrupted when -said fuel inlet is closed;

(d) a combustion air inlet communicating with said combustion chamber and with said vaporization chamber outlet for vaporized fuel from said vapori- Zation chamber outlet and combustion air to mix and the mixture of vaporized fuel and air to sustain combustion Within said combustion chamber and thus sustain said vaporization chamber heated upwardly within said fuel vaporization and carbon oxidation temperature range;

(e) an auxiliary heater in heat exchange relation to said vaporization chamber;

(f) temperature sensitive activation control means arranged for sensing temperature of said vaporization chamber and activating said auxiliary heater upon said vaporization chamber being cooled to a temperature downwardly Within said fuel vaporization carbon oxidation temperature range corresponding to a temperature attainable Within said range when said fuel control has said fuel inlet closed; and

( g) a purge air inlet in communication with said vaporization chamber for admitting purge air to said vaporization chamber to oxidize carbon while said fuel control has said fuel inlet close-d.

References Cited UNITED STATES PATENTS FREDERICK L. MATTESON, J R., Primary Examiner.

E. G. FAVORS, Assistant Examiner. 

1. APPARATUS FOR BURNING LIQUID FUEL, COMPRISING: (A) A COMBUSTION CHAMBER; (B) AN INSULATING MATERIAL ADJACENT TO SAID COMBUSTION CHAMBER; (C) A REFRACTORY MATERIAL LINING SAID COMBUSTION CHAMBER, SAID REFRACTORY MATERIAL BECOMING INCANDESCENT DURING COMBUSTION; (D) A VAPORIZER CHAMBER HAVING AN EXTERIOR SURFACE EXPOSED IN THE CEILING OF SAID COMBUSTION CHAMBER TO RADIATIONS FROM SAID REFRACTORY MATERIAL DURING COMBUSTION AND ISOLATED FROM CONVECTION CURRENTS IN SAID COMBUSTION CHAMBER BY BEING RECESSED INTO SAID INSULATING MATERIAL AND WITH THE REMAINING EXTERIOR SURFACES OF SAID VAPORIZER CHAMBER SURROUNDED BY SAID INSULATING MATERIAL; (E) A COMMUNICATION PASSAGE FROM SAID VAPORIZER CHAMBER TO SAID COMBUSTION CHAMBER; (F) A FUEL INLET FROM A FUEL SUPPLY SOURCE COMMUNICATING WITH SAID VAPORIZER CHAMBER; (G) A SOURCE OF AIR COMMUNICATING WITH SAID COMBUSTION CHAMBER FOR SUPPLYING COMBUSTION AIR TO MIX WITH VAPORIZED FUEL FROM SAID VAPORIZER CHAMBER; AND (H) IGNITION MEANS POSITIONED TO IGNITE THE MIXTURE OF VAPORIZED FUEL AND AIR. 